I have shown about two per 10 to the six mouse L cells spontaneously produce large amounts of interferon. Treatment of the cells with UV irradiation or ethylmethane sulphonate significantly increased this number. I propose to isolate human lymphoblastoid and fibroblast cells which spontaneously produce interferon and to characterize the products in terms of their antiviral, anticellular, and physical properties. In addition mouse L cells will be transformed with unfractionated human DNA in the presence of calcium phosphate and screened for cells spontaneously producing human interferon. Producing cells will be selected using suitable variations of the following technique which can detect fewer than one in a million producing cells: (1) A cell monolayer is covered with an agar overlay restricting the diffusion of any interferon to the immediate vicinity of the producing cell. (2) After a suitable time, the agar is removed and the cells infected with is-l, an interferon sensitive mutant of mengovirus previously isolated in my laboratory. This virus will kill all unprotected cells but spare those surrounding the producing cell, resulting in the formation of "protected centers" of live cells surrounded by a lawn of dead ones. (3) Protected centers are removed from the plate, and the interferon producing cells in their midst isolated by subsequent manipulations.